A fuel cell is a device that generates electricity by a chemical reaction. Among various fuel cells, solid oxide fuel cells use a hard, ceramic compound metal (e.g., calcium or zirconium) oxide as an electrolyte. Typically, in solid oxide fuel cells, an oxygen gas, such as O2, is reduced to oxygen ions (O2) at the cathode, and a fuel gas, such as H2 gas, is oxidized with the oxygen ions to form water at the anode. Fuel cells are generally designed as stacks, whereby subassemblies, each including a cathode, an anode and a solid electrolyte between the cathode and the anode, are assembled in series by locating an electrical interconnect between the cathode of one subassembly and the anode of another.
The efficiency of a solid oxide fuel cell (SOFC) depends in part on the distribution of oxygen and fuel in the cathode and anode, respectively. Gas delivery is typically provided to the SOFC stack by gas channels formed within the electrodes (cathode and anode). External access to the gas channels is typically provided by a manifold which acts as a conduit to supply fuel and oxygen into the gas channels. Manifolds can be configured as internal or external to the SOFC stack. External manifolds are simpler to fabricate and operate but have disadvantages including cracking of the seals between the manifold and the SOFC stack, causing gas leaks, due to mismatches in coefficient of thermal expansion (CTE) between the materials forming the manifold and the SOFC stack. Internal manifolds are more difficult to fabricate, but have sealing advantages over external manifolds discussed above, and also enable more external surface area of the SOFC stack to be exposed for more efficient heat transfer.
A frequent problem with internal manifolds is lack of uniform distribution of air and fuel flow across the respective electrodes. Due to gas flow dynamics, an internal manifold with equal diameter gas channels through the respective electrode generally delivers an uneven gas flow distribution through the electrode, with more gas flow through the channel closest to the manifold inlet, leading to non-uniform temperature distribution within the electrode, and lower efficiency of the SOFC stack.
Therefore, a need exists for an internal manifold design that overcomes or minimizes the above-mentioned problems.